It is our objective to carry out detailed structure analyses of enzymes by the established techniques of x-ray crystallography. We propose to continue studies of the flavoproteins involved in biological oxidations, already initiated by determination of the structures of flavodoxins from Clostridium MP and Anacystis nidulans. The semiquinone and reduced species of the latter flavodoxin will be analyzed and the structure of all three oxidation states defined more accurately by refinement and by extension of the resolution. Higher MW flavoproteins are also candidates for structural analysis. These include lactate oxidase and the complexes of ferredoxin-NADP-reductase with its protein substrates. A model of cytochrome c554 from A. nidulans will be constructed using the electron density map at 3 A resolution. Phases calculated by combining partial structure information with isomorphous replacement will be used to delineate portions of the molecule which are currently ill-defined. The analysis of iron superoxide dismutase from E. coli, based on data from two heavy atom isomorphs, will be extended from 5.9 A to higher resolution, and studies of inhibitor binding and metal replacement will be undertaken.